Optical pickup

ABSTRACT

An optical pickup can be provided, the optical pickup being capable of recording and playback of a plurality of optical disks having different specs by using light beams of different wavelengths, and further being suitable for integrating the semiconductor lasers and light receiving elements into a single package, by including: first and second semiconductor lasers adjacently disposed; a three-beam diffraction grating for generating three beams for tracking control; a second hologram element for diffracting light of the second semiconductor laser to guide it to a photosensor; a complex polarization beam splitter (PBS) for reflecting only light from the first semiconductor laser; and a first hologram element for diffracting light of the first semiconductor laser to guide it to the photosensor.

FIELD OF THE INVENTION

The present invention relates to an optical pickup which performsoptical recording and reproducing of information with respect toinformation recording media such as an optical disk and an optical card,and in particular to a compatible optical pickup applicable to aplurality of optical disks of different specifications for performingrecording and reproducing using light beams having differentwavelengths.

BACKGROUND OF THE INVENTION

In recent years, an optical disk has been utilized in a variety offields of audios, videos, computers and the like, because of itscapability of recording a large quantity of information signals withhigh density.

Particularly, optical disks having various different specifications(specs) such as CDs, CD-Rs and DVDs have been commercially available.What is required of an optical pickup is compatibility with these disksof the different specs so that a single optical pickup can performrecording or reproducing information of all types of disks.

In the case of CDs and CD-Rs, a substrate and/or a recording medium havecharacteristics which are optimized for an infrared light beam having awavelength in the vicinity of 780 nm. Similarly, in the case of DVDs,such characteristics are optimized for a red light beam having awavelength in the vicinity of 650 nm. Further, a development of arecording or reproducing disk for which the use of a blue light beam ofabout 400 nm would be available in future has been in progress.

An example of the optical pickup compatible with the disks for thusrecording and reproducing using different wavelengths is disclosed inJapanese Unexamined Patent Publication No. 128794/1997 (Tokukaihei9-128794 published on May 16, 1997), a configuration of which is shownin FIG. 37.

This optical pickup is provided with a first semiconductor laser 1, asecond semiconductor laser 2, a three-beam diffraction grating 3, alattice lens 4, an objective lens 5, a hologram element 7, and a lightreceiving element 8. The first semiconductor laser 1 starts oscillatingwhen a wavelength of laser light is in a 635 nm band, and the secondsemiconductor laser 2 starts oscillating when a wavelength of laserlight is in a 780 nm band. The three-beam diffraction grating 3 causes alight beam of each light sources to emerge as three beams which are usedfor tracking control. The lattice lens 4 acts as a concave lensdepending on a direction of a polarized wave of the light beam. Thehologram element 7 diffracts light reflected from a disk 6, therebyguiding it to the light receiving element 8.

Here, the first and second semiconductor lasers 1 and 2 disposed so thatthe directions of polarized waves thereof mutually intersect.

First, the following will explain an optical system in the case of usingthe first semiconductor laser 1 of the 635 nm band to play back anoptical disk having a plate thickness of 0.6 mm. Light emitted from thesemiconductor laser 1 is separated into three beams by the diffractiongrating 3 and transmitted through the hologram element 7, thereaftersimply passing through the inactive lattice lens 4 so as to converge onthe disk 6 by the objective lens 5.

The light reflected at the disk 6 and returned therefrom is similarlydiffracted at the hologram element 7, thereafter being guided to thelight receiving element 8. The light beams in the directions of thepolarized waves respectively have such lattice patterns as to be actedupon by the lattice lens 4.

Next, the following will explain an optical system in the case of usingthe second semiconductor laser 2 of the 780 nm band to play back anoptical disk having a plate thickness of 1.2 mm.

Light emitted from the semiconductor laser 2 is separated into threebeams by the diffraction grating 3 and transmitted through the hologramelement 7, thereafter receiving the concave lens action of the latticelens 4 and converging on the disk 6 by the objective lens 5.

The light reflected at the disk 6 and returned therefrom is similarlydiffracted at the hologram element 7, thereafter being guided to thelight receiving element 8. The light beams in the directions of thepolarized waves respectively have such lattice patterns as to be actedupon by the lattice lens 4.

Note that, it is designed that the concave lens action of the latticelens 4 corrects spherical aberration which emerges when a disk thicknessis in a range of 0.6 mm to 1.2 mm.

In this arrangement, in the case of the first semiconductor laser 1 forexample, the hologram element 7 is designed so that the diffractionlight of disk reflection light is guided to the light receiving element8.

Further, in the case of the second semiconductor laser 2 having anotherwavelength, it is arranged so that a focus point of the disk reflectionlight on the light receiving element 8, which tends to vary due todifferent diffraction angles formed by different wavelengths, is keptclose at a right position.

Further, both the light from the first semiconductor laser 1 and thelight from the second semiconductor laser 2 are respectively separatedinto three beams by the diffraction grating 3, and the same receivingelement 8 detects tracking error signals according to a three-beammethod.

With this arrangement, it is possible to commonly use the single lightreceiving element 8, two of which have been required conventionally,thereby reducing the number of components and the number of steps in theassembly.

In the case of the conventional optical pickup, with regard tosemiconductor laser light having a plurality of wavelengths, it isdesigned that a positional relationship among light sources is setaccording to a predetermined value, thereby guiding the light to theshared light receiving element by the single hologram element.

However, in the case where laser and the light receiving element areintegrated into one package, the laser and the light receiving elementare in general fixedly located at a predetermined position, that is, astem within the package. Therefore, it is often the case that thecontrol of a position and/or rotation is not available for the lightreceiving element when controlling the hologram element.

Namely, an offset control of a focus error signal and/or tracking errorsignal for example, which is caused by an error in the mounting of thelaser or the light receiving element, form tolerance in a phase on whichthe hologram element is mounted, or the like, is in most cases performedby the control of the hologram element alone. However, in that case,when optimizing the hologram element for one of semiconductor laserlight sources, it is very likely that the same optimum condition becomesineffectual when using the hologram element with another semiconductorlaser light source.

More specifically, controlling only the position of the hologram elementin the assembly raises problems such that servo error signals cannot beoptimized, or tolerances in the mounting of the laser and the lightreceiving element, in packaging, and the like are made highly exacting,thereby increasing costs.

Further, the hologram element is often provided with an aberrationcorrection function so as to obtain desirable light convergingcharacteristics on the light receiving element; however, it is difficultto design such a hologram pattern as to perform optimum aberrationcorrection with respect to a plurality of different wavelengths.

Furthermore, the conventional optical pickup has a problem that it isnot applicable to a plurality of optical disks of different specs inwhich different tracking error signals are used, respectively, becauseonly a tracking error signal according to the three-beam method can bedetected from either of light beams of the semiconductor laser having aplurality of waveforms.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical pickupwhich is compatible with a plurality of optical disks of different specsfor performing recording and reproducing using light beams of differentwavelengths, capable of an easy assembly control, and suitable fordownsizing integration.

In order to attain the foregoing object, the optical pickup according tothe present invention includes: a first light source for generating alight beam having a first wavelength; a second light source forgenerating a light beam having a second wavelength different from thefirst wavelength; a lens system for focusing the two light beams on anoptical disk; a photosensor for sensing reflection light beams from theoptical disk; an optical path splitting element for splitting opticalpaths of the two reflection light beams having different wavelengths;and first and second hologram elements for respectively diffracting atleast one of the two light beams of the different wavelengths which weresplit by the optical path splitting element, and guiding the thusdiffracted light to the shared photosensor.

With this arrangement, light beams emitted from the first and secondlight sources and having different wavelengths are focused on theoptical disk by the lens system, then, reflected. The optical paths ofthe respective reflecting light beams are split by the optical pathsplitting element, thereby travelling through different optical paths.Further, at least one of the reflecting light beams is diffracted at thefirst and second hologram elements, thereby guiding the both reflectinglight beams to the shared photosensor.

For example, the reflecting light beam of the first light source isdiffracted at the first hologram element, whereas the reflecting lightbeam of the second light source is diffracted at the second hologramelement, thereby guiding the both diffracted light beams to the sharedphotosensor through different optical paths. Further, in the case wherethe reflecting light beams of the first and second light sources arediffracted at the same first hologram element, since the light beamshave different wavelengths, and hence different diffraction angles, thediffraction light of either of the first and second light sources isdiffracted at the second hologram element, thereby guiding the bothreflecting light beams to the shared photosensor through differentoptical paths.

As explained, the reflecting light including light beams of differentwavelengths from the optical disk is split into different optical pathsby the optical path splitting element, and the thus split light beamsare diffracted at the first and second hologram elements, respectively,thereby guiding the light beams to the shared photosensor. This enablesthe shared use of the photosensor between the light beams havingdifferent wavelengths regardless of the positions of the first andsecond light sources. Consequently, it is possible to provide an opticalpickup which is capable of performing recording or reproducing withrespect to the plurality of optical disks having different specs whichare recorded and played back by using light beams of differentwavelengths, and further being suitable for integrating lasers and lightreceiving elements into a single package.

The optical pickup preferably has an arrangement in which the opticalpath splitting element and at least one of the first and second hologramelements are separately provided so that a position of either of the tworeflecting light beams can independently be controlled over thephotosensor by separately controlling each of the elements.

With this arrangement, when, for example, the optical path splittingelement transmits one of the reflecting light beams while reflecting theother, it is possible to correct deviation in the latter reflectinglight beam caused by the optical path splitting element by the followingcontrol: the hologram elements are first controlled so as to control theposition of the former reflecting light beam over the photosensor, then,the hologram elements are fixed, thereafter controlling the optical pathsplitting element so as to make the foregoing correction of deviation.Consequently, the hologram elements and/or the PBS can be controlledindependently with respect to the respective light sources, therebyeasily performing optimum assembly control with respect to light fromthe all light sources. This realizes milder tolerances in the mountingof the lasers and the light receiving elements and in the packagingprocess, thereby greatly reducing costs.

The foregoing optical pickup preferably has an arrangement in which thefirst hologram element is used to detect a tracking error signalaccording to a phase difference method or a push-pull method, and thesecond hologram element is used to detect a tracking error signalaccording to a three-beam method or a differential push-pull method.With this arrangement, it is possible to detect any of differenttracking error signals obtained by the foregoing tracking methodswithout changing the forms of the light receiving elements.

Additional objects, features, and strengths of the present inventionwill be made clear by the description below. Further, the advantages ofthe present invention will be evident from the following explanation inreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an optical system according to aFirst Embodiment of an optical pickup of the present invention.

FIG. 2 is a front view showing a configuration of an integrated laserunit according to the First Embodiment of the optical pickup of thepresent invention.

FIGS. 3(a) to 3(d) are explanatory views showing structures of adual-wave semiconductor laser element.

FIG. 4 is an explanatory view showing a relationship between a groovedepth and diffraction efficiency of a hologram.

FIG. 5 is an explanatory view showing a relationship between a groovedepth and reciprocating utilization efficiency of a hologram.

FIGS. 6(a) to 6(c) are explanatory views showing division patterns in afirst hologram element and a light receiving element, respectively.

FIG. 7 is an explanatory view showing division patterns in a secondhologram element and the light receiving element, respectively.

FIG. 8 is a diagram for an explanation of emergence of stray light inthe second hologram element.

FIG. 9 is a diagram for an explanation of emergence of stray light inthe first hologram element.

FIG. 10 is an explanatory view showing other division patterns in thefirst hologram element and the light receiving element, respectively.

FIG. 11 is an explanatory view showing other division patterns in thesecond hologram element and the light receiving element, respectively.

FIGS. 12(a) and 12(b) are explanatory views showing division patterns inthe hologram elements and the light receiving element for correcting anadverse effect of variations in wavelength of diffraction light.

FIGS. 13(a) and 13(b) are explanatory views showing other divisionpatterns in the hologram elements and the light receiving element forcorrecting an adverse effect of variations in wavelength of diffractionlight.

FIG. 14 is an explanatory view showing other division patterns of thefirst hologram element and the light receiving element.

FIG. 15 is an explanatory view showing other division patterns of thesecond hologram element and the light receiving element.

FIG. 16 is a diagram for an explanation of a method for controlling anintegrated unit.

FIG. 17 is a diagram for an explanation of control of the secondhologram element.

FIG. 18 is a diagram for an explanation of control of the secondhologram element.

FIG. 19 is a diagram for an explanation of control of a complex prism.

FIG. 20 is a diagram for an explanation of control of a complex prism.

FIG. 21 is a front view showing a configuration of a first integratedlaser unit according to a Second Embodiment of the optical pickup of thepresent invention.

FIG. 22 is a front view showing a configuration of a second integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 23 is a front view showing a configuration of a third integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 24 is a front view showing a configuration of a fourth integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 25 is a front view showing a configuration of a fifth integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 26 is a front view showing a configuration of a sixth integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 27 is a front view showing a configuration of a seventh integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 28 is a front view showing a configuration of an eighth integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 29 is a front view showing a configuration of a ninth integratedlaser unit according to the Second Embodiment of the optical pickup ofthe present invention.

FIG. 30 is a schematic diagram showing an optical system according to aThird Embodiment of the optical pickup of the present invention.

FIG. 31 is a front view showing a configuration of an integrated laserunit according to the Third Embodiment of the optical pickup of thepresent invention.

FIG. 32 is a diagram for an explanation of details of the integratedlaser unit according to the Third Embodiment of the optical pickup ofthe present invention.

FIG. 33 is an explanatory view showing division patterns and lightconverging states in the hologram element and the light receivingelement.

FIG. 34 is an explanatory view showing division patterns and other lightconverging states in the hologram element and the light receivingelement.

FIG. 35 is a schematic diagram showing an optical system according to aFourth Embodiment of the optical pickup of the present invention.

FIG. 36 is a schematic diagram showing an optical system according tothe Fourth Embodiment of the optical pickup of the present invention.

FIG. 37 is a schematic diagram showing an optical system in aconventional optical pickup.

DESCRIPTION OF THE EMBODIMENTS

The following will explain in detail a First Embodiment of the presentinvention with reference to FIGS. 1 through 20. Note that, componentsidentical with those shown in the conventional example above will begiven the same reference symbols, and explanation thereof will beomitted here.

FIGS. 1 and 2 are schematic diagrams of an optical pickup according tothe present embodiment. Light emitted from an integrated laser unit 10is changed into parallel light at a collimator lens 11 and transmittedthrough a wavelength selecting aperture 12, thereafter being focused onan optical disk 6 by an objective lens 5. The reflection light from theoptical disk 6 travels through same optical components as those of anoutward travel so as to be focused on a photosensor 27 of the integratedlaser unit 10.

Details of the integrated laser unit 10 will be discussed with referenceto FIG. 2. The integrated laser unit 10 includes a first semiconductorlaser 20 which starts oscillating when a wavelength of laser light is ina 650 nm band, and a second semiconductor laser 21 which startsoscillating when a wavelength of laser light is in a 780 nm band, whichare adjacently disposed.

Further, the integrated laser unit 10 is provided with a three-beamdiffraction grating 22, a first hologram element 23, a second hologramelement 24, a complex polarization beam splitter (complex PBS) 25, awave plate 26, and the photosensor 27, 30 or 31. The three-beamdiffraction grating 22 causes emergence of three beams for trackingcontrol. The first hologram element 23 diffracts a light beam of thefirst semiconductor laser 20 and guides the diffracted light beam to alight receiving element 27, and the second hologram element 24 diffractsa light beam of the semiconductor laser 21 and guides the diffractedlight beam to the light receiving element 27. The complex PBS 25 has apolarization beam splitter surface 25A and a reflector surface 25B.

Here, the first semiconductor laser 20, the second semiconductor laser21 and the light receiving element 27 are mounted inside a laser package28. The diffraction grating 22 and the first and second hologramelements 23 and 24 are respectively formed on the front and back of atransparent substrate 29. The transparent substrate 29, the complex PBS25 and the wave plate 26 are integrally and fixedly bonded to the laserpackage 28, thereby composing the integrated laser unit 10.

As the light sources of the optical pickup, the first semiconductorlaser 20 and the second semiconductor laser 21 are mounted. Thestructures of chips thereof include a ‘hybrid type’, a ‘monolithictype’, etc. The ‘hybrid type’ is a structure in which two types ofdifferently formed laser chips are fixed by thermal fusion, examples ofwhich are the horizontal placement of FIG. 3(a) and the verticalstacking of FIG. 3(b). The ‘monolithic type’ is a structure in which twotypes of laser chips are formed on the same substrate by repeatingcrystal growth twice in a direction of an active layer shown in FIG.3(c) and in a direction perpendicular to the active layer shown in FIG.3(d).

Generally, the ‘hybrid type’ fabricates a laser chip individually.Therefore, it is capable of combining laser chips having variouscharacteristics and waveforms, and can manage chip yields individually,thereby improving yields of the entirety of a dual-wave laser. However,the ‘hybrid type’ has a problem such that errors in an interval between,and in positions of, light emitting points may increase depending on atolerance at the time of fixedly setting the chip.

In comparison, the ‘monolithic type’ is combined with the limited typesof lasers, and has inferior yields. However, since two laser elementsare formed on the same substrate, only errors related to a semiconductorprocess may occur, thereby making it possible to very closely settolerances in positions of, and an interval between, the two laserelements.

Further, as shown in FIGS. 3(a) and 3(c), when the light emitting pointsare aligned side by side in the direction of the active layer, suchfabrication is easy, but an interval between the light emitting pointsincreases to an extent of 100 μm to 200 μm. This arises a problem suchthat an optic axis largely deviates from its right position when thesemiconductor laser chips thus fabricated are mounted on the pickup.

On the other hand, as shown in FIGS. 3(b) and 3(d), when the lightemitting points are aligned in the direction perpendicular to the activelayer, such fabrication is difficult, but an interval between the lightemitting points can greatly be reduced to an extent of a few μm to 20μm.

The optical pickup according to the present embodiment has anarrangement capable of controlling a sensing optical system individuallywith respect to each wavelength. Therefore, regardless of whether aninterval between the light emitting points and an error therein asexplained referring to FIGS. 3(a) to 3(d) are large or small, the lightsources having various chip structures can be utilized.

Next, the following will explain in detail a method for reproducinginformation of different optical disks. For example, when playing back aDVD having a plate thickness of 0.6 mm, a light beam 40 emitted from thefirst semiconductor laser 20 of the 650 nm band shown in FIGS. 3(a) to3(d) is transmitted through the diffraction grating 22, the secondhologram element 24, the polarization beam splitter surface 25A of thecomplex PBS 25, and the wave plate 26, and is changed into parallellight by the collimator lens 11, thereafter being transmitted throughthe wavelength selecting aperture 12 and focused on an optical disk 6Ahaving the plate thickness of 0.6 mm by the objective lens 5.

Further, returning light is transmitted through the objective lens 5,the wavelength selecting aperture 12 and the collimator lens 11, andreflected at the polarization beam splitter surface 25A and thereflector surface 25B, thereafter being diffracted at the first hologramelement 23 and focused on the photosensor 27.

Further, when playing back a CD having a plate thickness of 1.2 mm, alight beam 41 emitted from the second semiconductor laser 21 of the 780nm band is split into three beams by the diffraction grating 22, andtransmitted through the second hologram element 24, the polarizationbeam splitter surface 25A of the complex PBS 25, and the wave plate 26,and is changed into parallel light by the collimator lens 11, thereafterbeing given an aperture limit by the wavelength selecting aperture 12and focused on an optical disk 6B having the plate thickness of 1.2 mmby the objective lens 5.

Further, returning light is transmitted through the objective lens 5,the wavelength selecting aperture 12, the collimator lens 11 and thepolarization beam splitter surface 25A, and diffracted at the secondhologram element 24, then, focused on the photosensor 27.

The wavelength selecting aperture 12 has a wavelength selecting filmwhich allows transmission of, for example, the light of 650 nm. However,with respect to the light of 780 nm, it limits an aperture so that an NAof the objective lens 5 becomes 0.45.

Further, as to the objective lens 5, basically with respect to the lighthaving a wavelength of the 650 nm band and a NA 0.6, it takes anaspherical form such that an aberration is sufficiently reduced when theplate thickness is 0.6 mm. However, in the case of the light having awavelength of a 780 nm band, the objective lens 5 partially corrects itsform with respect to a light beam in a region in the vicinity of a NA0.45, where an aberration is large, so that the light is focused on anoptical disk having a plate thickness of 1.2 mm.

Accordingly, the objective lens 5 is designed so that the aberration issufficiently reduced with respect to respective light beams from twodifferent laser chips.

Next, the following will explain a relationship between a polarizationdirection and the complex PBS. One characteristic thereof will bediscussed. For example, the PBS surface 25A has a polarizationcharacteristic that it transmits almost 100% of P polarized light andreflects almost 100% of S polarized light with respect to both lighthaving a wavelength of the 650 nm band from the first semiconductorlaser 20 and light having a wavelength of the 780 nm band from thesecond semiconductor laser 21.

Further, the wave plate 26 is fixedly bonded to an upper surface of thecomplex PBS 25 and set to have a thickness which generates a phasedifference to act as a ¼ wave plate with respect to the wavelength ofthe 650 nm band of the first semiconductor laser 20, and as a ½ waveplate with respect to the wavelength of the 780 nm band of the secondsemiconductor laser 21.

When playing a DVD back, light (linearly polarized light in an xdirection in FIG. 1) which is the P polarized light emitted from thefirst semiconductor laser 20 is transmitted through the PBS surface 25Aand changed into circularly polarized light at the ¼ wave plate 26,thereafter being incident onto the optical disk 6A.

Returning light is incident again onto the ¼ wave plate 26, and changedinto linearly polarized light in a y direction (S polarized light) so asto be reflected at the PBS surface 25A and the reflector surface 25B.The reflected light is then incident onto the first hologram element 23and focused on the photosensor 27.

Consequently, since all the returning light from the optical disk 6A canbe guided to the photosensor, optical utilization efficiency can beimproved greatly.

Further, when playing a CD back, light (linearly polarized light in thex direction in FIG. 1) which is the P polarized light emitted from thesecond semiconductor laser 21, similarly, is transmitted through the PBSsurface 25A and changed into S polarized light (linearly polarized lightin the y direction in FIG. 1) at the ½ wave plate 26, thereafter beingincident onto an optical disk 6B.

Returning light is incident again onto the ½ wave plate 26 and restoredto the original P polarized light (linearly polarized light in the xdirection in FIG. 1), then, transmitted through the PBS surface 25A soas to be incident onto the second hologram element 24. Thereafter, theincident light is partially diffracted and focused on the photosensor27.

Further, another characteristic of the relationship between thepolarization direction and the complex PBS will be discussed. Forexample, the PBS surface 25A has a polarization characteristic asfollows: with respect to the light having the wavelength of the 650 nmband from the first semiconductor laser 20, the PBS surface 25Atransmits almost 100% of P polarized light and reflects almost 100% of Spolarized light; and with respect to the light having the wavelength ofthe 780 nm band from the second semiconductor laser 21, the PBS surface25A transmits almost 100% of both the P and S polarized light.

Further, the wave plate 26 is set to have a thickness which generates aphase difference to act as the ¼ wave plate with respect to thewavelength of the 650 nm band of the first semiconductor laser 20. Aphase difference is arbitrary with respect to the wavelength of the 780nm band of the second semiconductor laser 21.

For example, when playing a DVD back, P polarized light (linearlypolarized light in the x direction in FIG. 1) emitted from the firstsemiconductor laser 20 is transmitted through the PBS surface 25A andchanged into circularly polarized light at the ¼ wave plate 26,thereafter being incident onto the optical disk 6A.

Returning light is incident again onto the ¼ wave plate 26 and changedinto linearly polarized light in the y direction (S polarized light),then, reflected at the PBS surface 25A and the reflector surface 25B.The reflected light is then incident onto the first hologram element 23,thereafter being focused on the photosensor 27.

Accordingly, since all the returning light from the optical disk 6A canbe guided to the photosensor, optical utilization efficiency can beimproved greatly.

Further, when playing a CD back, P polarized light (linearly polarizedlight in the x direction in FIG. 1) emitted from the secondsemiconductor laser 21, similarly, is transmitted through the PBSsurface 25A and changed into elliptically polarized light (because of anarbitrary phase difference) at the wave plate 26, thereafter beingincident onto the optical disk 6B.

Returning light is incident again onto the wave plate 26 and changes itspolarization state. However, the PBS surface 25A transmits the light ofany wavelength emitted from the second semiconductor laser 21, and allthe returning light can thereby be incident onto the second hologramelement 24. The incident light is then partially diffracted and focusedon the photosensor 27.

In the integrated laser unit 10 according to the present embodiment,both of the two different characteristics of the relationship betweenthe polarization direction and the complex PBS can be utilized.

Next, the following will explain the structures and characteristics(wavelength selectivity) of the diffraction grating 22, and the firstand second hologram elements 23 and 24. The diffraction efficiency of arectangular hologram will be shown in FIGS. 4 and 5.

The diffraction efficiency of the rectangular hologram having the samegroove width and the land width can be given as follows:

when t is a groove depth, λ is a wavelength, and n is a refractive indexof the transparent substrate,0th order diffraction efficiency (transmissivity) η₀=(cos Δø)²±1st order diffraction efficiency η₁=(2/π×sin Δø)²therefore,Δφ=πt(n−1)/λ.

FIG. 4 shows a relationship between 0th/±1st order diffractionefficiency and a groove depth when wavelengths are 650 nm and 780 nm.Further, FIG. 5 shows a relationship between the product of 0th orderdiffraction efficiency and ±1st order diffraction efficiency(reciprocating utilization efficiency) and a groove depth. Here, assumethat hologram glass is a quartz having n=1.457 (λ=650 nm), and n=1.454(λ=780 nm).

The three-beam diffraction grating 22 having a groove depth of about 1.4μm will be discussed referring to FIG. 4. For example, when quartz glassis adopted, with respect to the light having the wavelength of the 780nm band, a main beam (0th order transmissivity) is 72%, and a sub-beam(±1st order diffraction efficiency) is 12%, thereby obtaining a ratio oflight quantities of three beams, which is sub:main:sub=1:6:1.

In addition, here, with respect to the light having the wavelength ofthe 650 nm band, the diffraction efficiency of ±1st order light issubstantially 0, thereby being scarcely affective.

The second hologram element 24 requires to secure a quantity of lightincident onto the photosensor 27 with respect to the light having thewavelength of the 780 nm band. Likewise, with respect to the lighthaving the wavelength of the 650 nm band, the second hologram element 24is required to secure a quantity of light incident onto the optical disk6, thereby setting a groove depth at about 0.35 μm.

As shown in FIGS. 4 and 5, with respect to the light having thewavelength of the 780 nm band, 0th order is 65%, ±1st order is 14%, andreciprocating utilization efficiency is about 9%, thereby securing avalue of maximum efficiency in the vicinity of 10%. In that case, withrespect to the light having the wavelength of the 650 nm band, 0th ordertransmission decreases to about 50%.

Since only the light having the wavelength of the 650 nm band isincident onto the first hologram element 23, in order to secure as largea quantity of light incident onto the photosensor 27 as possible, asshown in FIG. 4, a groove depth is set at about 0.7 μm, therebyattaining ±1st order diffraction efficiency of about 40%.

Note that, with regard to the light having the wavelength of the 650 nmband from the first semiconductor laser 20, it is reduced to about 50%by the second hologram element 24 when travelling outward. However,returning light from the optical disk 6 is diffracted by 40% at thefirst hologram element 23. Therefore, reciprocating utilizationefficiency is about 20% in its product, thereby attaining higherefficiency than the maximum reciprocating utilization efficiency of 10%shown in FIG. 5.

Next, the following will explain the structures of the first hologramelement 23, the second hologram element 24 and the photosensor 27, and aservo signal detecting method. FIGS. 6(a) to 6(c) show the firsthologram element 23 and the form of the light receiving element of thephotosensor 27.

As shown in FIG. 6(a), the hologram element 23 is split into threeregions 23 a to 23 c by a split line 23 l in the x directioncorresponding to a direction of the radius (hereinafter, “radialdirection”) of the optical disk 6, and by a split line 23 m in the ydirection corresponding to a direction of a track (hereinafter, “trackdirection”) of the optical disk 6.

The light receiving element is made up of a 2-division light receivingelement, one half of which is a light receiving region 27 a and theother a light receiving region 27 b divided by a division line 27 l, andfour light receiving regions 27 c to 27 f (here, respective output fromthe light receiving regions are referred to as Sa to Sf).

For example, when playing a DVD back, returning light from the opticaldisk 6A which is originally emitted from the first semiconductor laser20 is incident onto the first hologram element 23.

When a light beam is focused by the objective lens 5 on an informationrecording side of the optical disk 6A, one of all incident beams whichwas diffracted at a region 23 of the hologram element 23 is focused onthe division line 27 l dividing the light receiving element into halves27 a and 27 b. Diffraction light in a region 23 b of the hologramelement 23 is focused on the light receiving region 27 c. Likewise,diffraction light in a region 23 c of the hologram element 23 is focusedon the light receiving region 27 d.

When the optical disk 6A and the objective lens 5 approach each other, aresultant state is as shown in FIG. 6(b). On the other hand, when theydepart from each other, a resultant state is as shown in FIG. 6(c).Therefore, using the respective output Sa and Sb from the lightreceiving regions 27 a and 27 b, it is possible to detect a focus errorsignal (FES) according to a single knife edge method by FES=Sa−Sb.

Further, when playing back the optical disk 6A storing pit information,a change in a signal phase difference between the respective output Scand Sd from the light receiving regions 27 c and 27 d is detected first,thereby detecting a tracking error signal 1 (TES1) according to a phasedifference (DPD) method.

In the case of the optical disk 6A having grooves formed therein, it ispossible to detect a tracking error signal 2 (TES2) according to apush-pull method by TES2=Sc−Sd.

In addition, a stored information signal (RF signal) can be reproducedby RF=Sa+Sb+Sc+Sd.

Next, the following will explain the second hologram element 24 and thelight receiving element of the photosensor 27 with reference to FIG. 7.As shown in FIG. 7, the hologram element 24 is split into two regions 24a and 24 b by a split line 24 l in the x direction corresponding to theradial direction of the optical disk. Here, the light receiving elementis the same as the aforesaid photosensor 27.

When playing a CD back, returning light from the optical disk 6B whichis originally emitted from the second semiconductor laser 21 is incidentonto the second hologram element 24.

When a light beam is focused by the objective lens 5 on an informationrecording side of the optical disk 6B, one of all incident beams whichwas diffracted at a region 24 of the second hologram element 24 isfocused on the division line 27 l dividing the light receiving elementinto halves 27 a and 27 b, and light diffracted at a region 24 b of thesecond hologram element 24 is focused on the light receiving region 27c.

The light of the semiconductor laser 21 is split into a main beam andtwo sub-beams A and B by the three-beam diffraction grating 22.Therefore, the sub-beam A diffracted at the regions 24 a and 24 b of thesecond hologram element 24 is focused on the light receiving region 27f, while the sub-beam B diffracted at the regions 24 a and 24 b of thesecond hologram element 24 is focused on the light receiving region 27e.

A focus error signal (FES), as with the DVD, can be detected byFES=Sa−Sb.

Further, a stored information signal (RF signal) can be reproduced byRF=Sa+Sb+Sc.

By thus using the partially shared light receiving element for thedifferent optical disks 6A and 6B, a servo signal and an RF signal canbe detected.

Next, the following will explain a pattern of another light receivingelement. FIGS. 8 and 9 show the problem of stray light that may possiblyemerge with respect to light receiving elements 7 a to 7 f.

The optical pickup according to the present embodiment as describedreferring to FIG. 2 can, in principle, completely separate optical pathsof light from the two laser-light sources of different waveforms at thePBS. However, depending on a tolerance and/or variations in wavelengthwith regard to separating film characteristics and wave platecharacteristics of the PBS, light which failed to be separated at thePBS and leaks in a different optical path, i.e., so-called ‘stray light’may possibly emerge.

FIG. 8 shows stray light which is incident from the second hologramelement 24 when the first hologram element 23 is used to detect asignal.

As with FIG. 6(a), light diffracted at the region 23 a of the hologramelement 23 is focused on the division line 27 l dividing the lightreceiving element into halves 27 a and 27 b, and light diffracted at theregions 23 b and 23 c of the hologram element 23 are respectivelyfocused on the light receiving regions 27 c and 27 d. However, part ofthe light transmitted through the PBS surface 25A shown in FIG. 2 isdiffracted at the second hologram element 24, thereby generatingdiffraction light 45 a and 45 b in the regions 24 a and 24 b,respectively.

The second hologram element 24 is designed for the light having thewavelength of the 780 nm band. Therefore, with respect to the lighthaving the wavelength of the 650 nm band, its diffraction angle becomessmaller than a designed angle, and the light is focused on a positioncloser to a hologram than the expected position.

For example, when it is designed so that the diffraction light 45 adeviates off the light receiving regions 27 a and 27 b, as shown in FIG.8, the diffraction light 45 a is incident onto the light receivingregion 27 c, thereby possibly causing adverse effects such as noise andoffset.

Further, FIG. 9 shows stray light incident from the first hologramelement 23 in the case of detecting a signal in the second hologramelement 24.

As with FIG. 7, when a main beam and sub-beams A and B from the regions24 a and 24 b of the second hologram element 24 are focused on theprimarily expected position, part of light reflected at the PBS surface25A in FIG. 2 is diffracted at the first hologram element 23, therebygenerating diffraction light beams 46 a to 46 i in the regions 23 a to23 c of the first hologram element 23.

The first hologram element 23 is designed for the light having thewavelength of the 650 nm band. Therefore, with respect to the lighthaving the wavelength of the 780 nm band, its diffraction angle becomeslarger than a designed angle, and the light is focused on a positionmore distant from a hologram than the expected position. In addition,since the light is split into three beams by the diffraction grating 22shown in FIG. 2, stray light is generated from the sub-beams.

For example, when it is designed so that the diffraction light 46 a and46 c deviate off the light receiving regions 27 a, 27 b and 27 d, asshown in FIG. 9, diffraction light beams 46 e, 46 d and 46 i arerespectively incident onto the light receiving region 27 c which sensesthe main beam, and the light receiving regions 27 e and 27 f which sensethe sub-beams, thereby possibly causing adverse effects.

Therefore, by using light receiving elements as shown in FIGS. 10 and11, the adverse effects can be removed. The light receiving elements arealigned in a row in a direction perpendicular to a direction in whichthe hologram elements 23 and 24 are aligned. In this arrangement, straylight from the other hologram element is prevented.

FIG. 10 shows the first hologram element 23 and the form of lightreceiving elements of a photosensor 30. The division state of thehologram element 23 is the same as that of FIG. 6(a). The lightreceiving elements is made up of a 2-division light receiving elementwhich is divided into light receiving regions 30 a and 30 b by adivision line 30 l, and six light receiving regions 30 c to 30 h (here,respective output from the light receiving regions are referred to as Sato Sh).

For example, when playing a DVD back, light diffracted at the region 23a of the first hologram element 23 is focused on the division line 30 ldividing the light receiving element into the light receiving regions 30a and 30 b. The light diffracted at the region 23 b of the firsthologram element 23 is focused on the light receiving region 30 c, andthe light diffracted at the region 23 c of the first hologram element 23is focused on the light receiving region 30 d. A servo signal isdetected from absolutely the same calculation described referring toFIGS. 6(a) to 6(c).

Next, FIG. 11 shows the second hologram element 24 and the lightreceiving elements of the photosensor 30. As shown in FIG. 11, in thecase of the hologram element 24, as with FIG. 7, light diffracted at theregion 24 a of the second hologram element 24 is focused on the divisionline 30 l dividing the light receiving element into light receivingregions 30 a and 30 b, and light diffracted at the region 24 b of thesecond hologram element 24 is focused on the light receiving region 30c.

The sub-beam A diffracted at the regions 24 a and 24 b of the secondhologram element 24 is focused on the light receiving regions 30 f and30 h, respectively. The sub-beam B diffracted at the regions 24 a and 24b of the second hologram element 24 is focused on the light receivingregions 30 e and 30 g, respectively.

A focus error signal (FES), as with the DVD, can be detected byFES=Sa−Sb.

Further, a tracking error signal 3 (TES 3), according to the three-beammethod, can be detected by TES 3=(Sf+Sh)−(Se+Sg).

Further, a stored information signal (RF signal) can be reproduced byRF=Sa+Sb+Sc.

By thus aligning light receiving elements in a row, an adverse effect ofstray light from the other hologram element can be removed.

Next, the following will explain a method for controlling generation ofan FES offset due to variations in wavelength. In FES detectionutilizing diffraction light by a hologram element and the like, avariation in wavelength of a light source causes a diffraction angle tochange, then, a beam position on the light receiving element shifts,thereby causing a phenomenon of an offset.

A popular method for correcting the offset is such that, for example, anangle is added to a direction of the division line of the 2-divisionlight receiving element and in a diffracting direction of a beam. In thepresent embodiment, in order to detect an FES, which is caused bydiffraction light from the two hologram elements, by the shared2-division light receiving element, variations in wavelength of bothlight beams should be corrected.

In FIG. 12(a), in order to correct an adverse effect of variations inwavelength of light from the first hologram element 23 first, a focusingposition of the diffraction light from the region 23 a of the firsthologram element 23 is shifted into a positive y direction by a quantityL1.

This produces an angle between a diffracting direction k and thedivision line 27 l. Therefore, there occurs no difference in outputbetween the light receiving regions 27 a and 27 b even when variationsin wavelength cause a beam to shift, thereby generating no offset in anFES according to the single knife edge method.

FIG. 12(b) shows light from the second hologram element 24. In order toachieve the same effect as above, it is effective to use a semicircularbeam from the region 24 a of the second hologram element 24 so as togenerate an FES according to the single knife edge method.

According to the single knife edge method, an FES can be detected byusing either of division patterns of the regions 24 a and 24 b of thesecond hologram element 24. However, in the case where a focusingposition, that is, a position of the division line 27 l is shifted intothe positive y direction, only the use of the semicircular beam from theregion 24 a of the second hologram element 24 is effective in thecancellation of the FES offset due to variations in wavelength.

An optimum value of the shift quantity L1 can be calculated bycontrolling distances L2 and L3 between a focusing point and the centerof a hologram.

Further, in FIG. 13(a), in order to correct an adverse effect ofvariations in wavelength of light from the first hologram element 23,the division line 27 l on which the diffraction light from the region 23a of the first hologram element 23 is focused is tilted by a quantity θ.

This prevents occurrence of a difference in output between the lightreceiving regions 27 a and 27 b even when variations in wavelength causea beam to shift, thereby generating no offset in an FES according to thesingle knife edge method.

FIG. 13(b) shows light from the second hologram element 24. In order toachieve the same effect as above, unlike the case of FIG. 12(b), asshown in FIG. 13(b), it is necessary to use a semicircular beam from theregion 24 b of the second hologram element 24. An optimum value of thetilt angle θ can be calculated by controlling the distances L2 and L3between a focusing point and the center of a hologram.

As discussed, even when adopting the light receiving element common tothe both two hologram elements 23 and 24, an offset by variations inwavelength of an FES due to diffraction light from the hologram elements23 and 24 can be corrected.

Note that, in FIGS. 7 and 11, the three-beam method has been adopted inTES detection for a CD; however, a method is not limited to this, and itis possible to detect a TES according to a differential push-pull (DPP)method which also uses three beams. This method is used in arecording/playback pickup optical system for CD-Rs, etc.

FIGS. 14 and 15 show hologram elements and the light receiving elementsof a photosensor 31. FIG. 14 shows the first hologram element 23 and theform of the light receiving elements of the photosensor 31. As shown inFIG. 14, the hologram element 23 is divided into three regions as withFIG. 6(a). The light receiving elements is made up of a 2-division lightreceiving element which is divided into light receiving regions 31 a and31 b by a division line 31 l, and six light receiving regions 31 c to 31h (here, respective output from the light receiving regions are referredto as Sa to Sh).

For example, when playing a DVD back, light diffracted at the region 23a of the hologram element 23 is focused on the division line 31 ldividing the light receiving element into the light receiving regions 31a and 31 b. Likewise, light diffracted at the region 23 b of thehologram element 23 is focused on the light receiving region 31 c, andlight diffracted at the region 23 c of the hologram element 23 isfocused on the light receiving region 31 d. A servo signal and an RFsignal can be calculated by the absolutely the same calculationdescribed referring to FIGS. 6(a) to 6(c).

Next, FIG. 15 shows the second hologram element 24 and the lightreceiving elements of the photosensor 31. As shown in FIG. 15, thehologram element 24 is split into three regions 24 a to 24 c by a splitline 24 l in the x direction corresponding to the radial direction ofthe optical disk and a split line 24 m in the y direction correspondingto the track direction.

When playing a CD back, light diffracted at the region 24 a of thesecond hologram element 24 is focused on the division line 31 l dividingthe light receiving element into the light receiving regions 31 a and 31b. Likewise, light diffracted at the region 24 b of the second hologramelement 24 is focused on the light receiving region 31 d, and lightdiffracted at the region 24 c of the second hologram element 24 isfocused on the light receiving region 31 c.

Since light from the second semiconductor laser 21 is split into a mainbeam and two sub-beams A and B by the three-beam diffraction grating 22,the sub-beams A and B diffracted at the region 24 c of the secondhologram element 24 are respectively focused on the light receivingregions 31 f and 31 e. The sub-beams A and B diffracted at the region 24b of the second hologram element 24 are respectively focused on thelight receiving regions 31 h and 31 g.

A focus error signal (FES), as with the DVD, can be detected byFES=Sa−Sb.

Further, a tracking error signal 4 (TES 4) can be detected according tothe differential push-pull (DPP) method, using push-pull signals TES 5,TES (A) and TES (B) of the main beam and the sub-beams A and B,respectively, by4=TES 5−k·(TES (A)+TES (B))=(Sa−Sb)−k·((Sh−Sg)+(Sf−Se)).

Here, a coefficient k is provided to correct a difference in lightintensity between the main beam and sub-beams. Accordingly, when a ratioof intensity is the main beam:sub-beam A:sub-beam B=a:b:b, thecoefficient k=a/(2b).

Further, a stored information signal (RF signal) can be reproduced byRF=Sa+Sb+Sc+Sd.

Next, the following will explain the hologram elements and the controlof the complex PBS. Explained first will be the offset control of an FESthat is important in the assembly control of the integrated laser unit10.

FIG. 16 is a three-dimensional view of the integrated laser unit 10shown in FIG. 2. In this unit, the semiconductor lasers 20 and 21, andthe photosensor 27 are fixedly located at a predetermined position on astem, which is not shown, within the laser package 28. On an uppersurface of the laser package 28 are stacked the transparent substrate 29having holograms 23, 24 and the diffraction grating 22 formed thereinand complex PBS 25. The control is performed by parallel shifts in the Xand Y directions and rotation around a central Z axis in a θ direction,within a plane (within a plane XY) which is perpendicular to an emissionlight axis.

Control procedures will be explained briefly. First, the secondsemiconductor laser 21 is caused to emit light, and when returned, thereturning light is diffracted at the second hologram element 24, therebyperforming control in a state that the thus diffracted light is guidedto the photosensor 27. In that case, in order that the FES offsetbecomes zero, the transparent substrate 29 (i.e., the second hologramelement 24) is controlled by rotation in the θ direction around an opticaxis O which is shown in FIG. 16, while controlling positions in the Xand Y directions so that the center of the beam and the center of thehologram coincide.

Here, the returning light is only transmitted through the PBS surface25A of the complex PBS 25. Therefore, a beam over the photosensor 27 isnot affected regardless of whether the complex PBS 25 is rotatedtogether with the transparent substrate 29 or fixed without beingrotated.

Next, after the transparent substrate 29 is fixedly bonded to the laserpackage 28, the first semiconductor laser 20 is caused to emit light,and the returning light is reflected at the reflector surfaces 25A and25B of the complex PBS and diffracted at the first hologram element 23,thereafter guiding the thus diffracted light to the photosensor 27.

Since the transparent substrate 29 is controlled by rotation whencontrolling the second semiconductor laser 21, the position of the firsthologram element 23 deviates from the primary position accordingly.Therefore, next, the same control by rotation in the θ direction aroundthe optic axis O is performed with respect to the complex PBS 25,thereby controlling the FES offset to zero.

Here, the control of the parallel shifts of the complex PBS 25 into theX and Y directions is not affected by the shift of a beam, therebyrequiring no fine adjustment.

As described, with respect to beams from the two semiconductor lasers 20and 21, the position control over the photosensor 27 can be performedindependently and separately. Accordingly, even when there aretolerances in the positional relationship between laser chips or in theposition and/or the angle of the laser package 28, the photosensor 27,the hologram elements 23 and 24, and/or the complex PBS 25, an optimumservo error signal can be obtained by the control.

Furthermore, the FES offset control will be explained in detail withreference to FIGS. 17 to 20. FIGS. 17 to 20 schematically showpositional relationships among the hologram elements 23 and 24, and thelight receiving regions 30 a to 30 h, where the hologram elements andthe light receiving elements are the same as those shown in FIG. 11.

FIG. 17 shows the returning light of the second semiconductor laser 21,where, one of the returning light beams 43 diffracted at a semicircularregion 24 a of the hologram element 24 is directed to a point in thevicinity of the division line 30 l on an FES detecting 2-division lightreceiving element.

However, since component errors, etc., cause a relative position amongthe hologram, the laser chip and the light receiving elements to deviatefrom a designed value in a range of a tolerance, the position of thebeam deviates off the division line, and/or deviates from the focusingstate, thereby upsizing the beam.

Accordingly, as shown in FIG. 18, in order that the FES offset becomeszero in that state, the hologram element 24 is rotated so thatdiffraction light from the region 24 a is directed to the division line30 l, thereby reducing the FES offset to zero.

Next, the control of light from the first semiconductor laser 20 will beexplained with reference to FIGS. 19 and 20. FIG. 19 shows a state inwhich returning light from the second semiconductor laser 21 iscontrolled, where the first hologram element 23 is shifted from theprimary position (indicated by the dotted line).

Accordingly, the center of the returning light 42 reflected at thecomplex PBS 25 deviates from the center of the first hologram element23. Further, light diffracted at the region 23 a of the first hologramelement 23 for the FES detection deviates off the division line 30 l ofthe FES detecting 2-division light receiving element, or deviates fromthe focusing state, thereby upsizing the beam.

Therefore, in order that the FES offset becomes zero in that state, thecomplex PBS 25 is rotated around the optic axis O this time. This shiftsa beam on the hologram 23 as shown in FIG. 20, that is, shifts theposition of the beam on the light receiving element so as to controldiffraction light from the region 23 a of the first hologram element 23to be directed to the division line 30 l, thereby reducing the FESoffset to zero.

Note that, in the embodiment above, explanation has been made throughthe case where a red laser of a 650 nm band is adopted as the firstsemiconductor laser 20, and an infrared laser of a 780 nm band isadopted as the second semiconductor laser 21; however, the presentinvention is not limited to that case and is also applicable to the casewhere laser sources of two different wavelengths including a blue laserof a 400 nm band is adopted.

Next, the following will describe a Second Embodiment of the presentinvention with reference to FIGS. 21 to 29. Note that, the same portionsas those pertaining to the First Embodiment will be given the samereference symbols, and explanation thereof will be omitted here.

In the First Embodiment, as shown in FIGS. 1 and 2, the second hologramelement 24 was provided in the middle of the outward travel paths oflight beams 40 and 41 respectively emitted from the first and secondsemiconductor lasers 20 and 21 with respect to the optical disk 6. Thisarrangement involves a problem of the large loss of a quantity of lightincident onto the optical disk 6 because unwanted ±1st order diffractionlight is generated by the second hologram element 24 even in the outwardtravel paths. Further, in respect of light of a second wavelength, aquantity of light incident on the photosensor 27 is determined by theproduct of the 0th order diffraction efficiency and the ±1st orderdiffraction efficiency of the second hologram element 24, thereby makingit difficult to increase a quantity of detection light. The SecondEmbodiment of the present invention is the invention to solve suchproblems.

FIGS. 21 and 22 are schematic views of an optical pickup according tothe present embodiment. Here, the optical pickup is arranged as withFIG. 1 and the First Embodiment above except for the configuration ofthe integrated laser unit 10, and detailed explanation of the opticalpickup will be omitted. An integrated laser unit 10 will be explainedwith reference to FIG. 21.

The integrated laser unit 10 according to the present embodimentincludes a first semiconductor laser 20, a second semiconductor laser21, a three-beam diffraction grating 22, a complex PBS 25, a wave plate26, a first hologram element 23, a second hologram element 24, and alight receiving element 27. The first semiconductor laser 20 whichstarts oscillating when a wavelength of laser light is in a 650 nm bandand the second semiconductor laser 21 which starts oscillating when awavelength of laser light is in a 780 nm band are adjacently disposed.The three-beam diffraction grating 22 causes emergence of three beamsfor tracking control. The complex PBS 25 has a polarization beamsplitter surface 25A and a reflector surface 25B. The first hologramelement 23 diffracts a light beam of the first semiconductor laser 20and guides the diffracted light beam to the light receiving element 27,and the second hologram element 24 diffracts a light beam of thesemiconductor laser 21 and guides the diffracted light beam to the lightreceiving element 27.

Further, the first hologram element 23 is formed on a transparentsubstrate 291, to which it is fixedly bonded integrally with the complexPBS. The second hologram element 24 is formed on an upper side of atransparent substrate 292. The structures of light sources of theoptical pickup are the same as those described in the First Embodimentwith reference to FIG. 3, thereby omitting explanation thereof here.

Next, the following will explain a method for playing back differentoptical disks. Since this is basically the same as the First Embodiment,the function of the integrated laser unit 10 alone will be explainedhere.

For example, when playing back a DVD having a plate thickness of 0.6 mm,a light beam 40 emitted from the first semiconductor laser 20 of the 650nm band is transmitted through the diffraction grating 22, then furthertransmitted through the polarization beam splitter surface 25A of thecomplex PBS 25 and the wave plate 26, thereafter being focused on anoptical disk 6A having a plate thickness of 0.6 mm by a collimator lens11 and an objective lens 5.

Further, returning light is reflected at the polarization beam splittersurface 25A and the reflector surface 25B, thereafter being diffractedat the first hologram element 23, transmitted through the secondhologram element 24, then, focused on a photosensor 27.

Meanwhile, when playing back a CD having a plate thickness of 1.2 mm, alight beam 41 emitted from the second semiconductor laser element 21 ofthe 780 nm band is split into three beams by the diffraction grating 22.The split beams are transmitted through the polarization beam splittersurface 25A of the complex PBS 25 and the wave plate 26, thereafterbeing focused on an optical disk 6B having a plate thickness of 1.2 mmby the collimator lens 11 and the objective lens 5.

Further, returning light is reflected at the polarization beam splittersurface 25A and the reflector surface 25B, thereafter being transmittedthrough the first hologram element 23, diffracted at the second hologramelement 24, then, focused on the photosensor 27.

Here, the same function of the three-beam diffraction grating 22 as withthe First Embodiment is utilized.

It is desirable that the first hologram element 23 is set to have agroove depth such that ±1st diffraction efficiency is high with respectto the light of the wavelength of the first semiconductor laser 20, and0th order efficiency is high with respect to the light of the wavelengthof the second semiconductor laser 21. On the contrary, it is desirablethat the second hologram element 24 is set to have a groove depth suchthat ±1st diffraction efficiency is high with respect to the light ofthe wavelength of the second semiconductor laser 21, and 0th orderefficiency is high with respect to the light of the wavelength of thefirst semiconductor laser 20.

Further, though respective diffraction angles of the hologram elementsincrease, it is possible to ease the above conditions of the groovedepths. For example, as shown in FIG. 22, it may be arranged such thatthe second hologram element 24 is formed on a lower side of thetransparent substrate 292 so that the light of the first semiconductorlaser 20 diffracted at the first hologram element 23 is prevented frompassing through the second hologram element 24.

A polarization characteristic of the PBS surface 25A of the complex PBS25 is such that, as with the example in the First Embodiment, 100% of Ppolarized light is transmitted and substantially 100% of S polarizedlight is reflected in both cases of the light from the firstsemiconductor laser 20 having the wavelength of the 650 nm band and thelight from the second semiconductor laser 21 having the wavelength ofthe 780 nm band.

Further, the wave plate 26 is fixedly bonded to an upper side of thecomplex PBS 25 and set to have a thickness which generates a phasedifference to act as a ¼ wave plate with respect to both the light fromthe first semiconductor laser 20 having the wavelength of the 650 nmband and the light from the second semiconductor laser 21 having thewavelength of the 780 nm band.

Accordingly, respective P polarized light (linearly polarized light inthe x direction in FIG. 1) 40 and 41 emitted from the both light sourcesis all changed into circularly polarized light at the ¼ wave plate 26,thereafter being incident on the optical disk 6A. The returning light isincident again on the ¼ wave plate 26 so as to be changed into linearlypolarized light in the y direction (S polarized light). The linearlypolarized light is reflected at the PBS surface 25A and the reflectorsurface 25B, thereafter being incident on each of the hologram elements23 and 24.

Note that, the structures of the first hologram element 23 and thephotosensor 27, and the method for detecting a servo signal as explainedwith reference to FIGS. 6, 7, 10, 11, 14 and 15 are applicable, therebyomitting explanation thereof here. In addition, the absolutely the samedetection method adopted in the First Embodiment is applicable to thesecond hologram element 24 here, except for its location within anoptical path from the first hologram element 23 to the photosensor 27,thereby omitting explanation here.

Next, the control of the hologram elements will be explained. Inaddition, as with the First Embodiment, the FES offset control will beexplained. However, since they are the same as those described in theFirst Embodiment, explanation will be made briefly here.

First, the first semiconductor laser 20 is caused to emit light, and thereturning light is diffracted at the first hologram element 23 andguided to the photosensor 27, thereafter performing control in thatstate. In order that the FES offset here becomes zero, the transparentsubstrate 291 (i.e., the first hologram element 23) to which the complexPBS 25 is integrally and fixedly bonded is controlled by rotation in theθ direction shown in FIG. 16, while controlling the positions in the xand y direction so that the center of a beam and the center of ahologram coincide.

In that case, since the light 42 is only transmitted through the secondhologram element 24, the position of the transparent substrate 292 isnot affected.

Further, the second semiconductor laser element 21 is caused to emitlight while fixing the position of the transparent substrate 291 towhich the complex PBS 25 is integrally and fixedly bonded. The returninglight is diffracted at the second hologram element 24, thereafterguiding the diffracted light to the photosensor 27. Since the complexPBS 25 has been optimized for the first semiconductor laser 20 togetherwith the first hologram element 23, the complex PBS 25 may not ideallybe suited to the light of the second semiconductor laser 21.

Accordingly, in order to reduce the FES offset to zero, the transparentsubstrate 292 (i.e., the second hologram element 24) is controlled byrotation in the θ direction, while controlling the positions in the xand y directions so that the center of a beam and the center of ahologram coincide.

As described, it is possible to control the positions of beamsrespectively from the two semiconductor lasers 20 and 21 over thephotosensor 27 independently and separately. This makes it possible toobtain an optimum servo error signal by the control even when there aretolerances in the positional relationship between laser chips, or in theposition or angle of the laser package 28, the photosensor 27, thehologram elements 23 and 24, and/or the complex PBS 25.

Meanwhile, unlike the First Embodiment, the hologram element 24 is notprovided in the middle of an outward travel path. Therefore, anyunwanted diffraction light is not generated in the outward travel path,thereby increasing a quantity of emission light from the objective lens5. Moreover, providing the hologram elements 23 and 24 only in a returntravel path enables diffraction efficiency to be set high, therebyincreasing a quantity of detected light. This arrangement isparticularly effective to a recording optical pickup which requires alarger quantity of emission light from the objective lens.

Next, the following will explain details of another configurationaccording to the Second Embodiment of the present invention, withreference to FIG. 23. Note that, the same portions as those of the aboveconfiguration of FIG. 21 will be given the same reference symbols, andexplanation thereof will be omitted here.

In the integrated laser unit 10 of FIGS. 21 and 22, in the case ofcontrolling the first and second hologram elements 23 and 24, thecomplex PBS 25 and the first hologram element 23 on the upper side arefirst controlled integrally. Thereafter, the second hologram element 24on the lower side should be controlled while fixing the complex PBS 25and the first hologram element 23 thus integrally controlled lest theyshould move. However, such control poses problems in terms offabrication that devices such as controlling tools become complicated,and stricter accuracy is demanded. Therefore, a configuration to solvethese problems will be described below.

FIG. 23 shows an integrated laser unit 10 having another configurationaccording to the present embodiment, which differs from the integratedlaser unit 10 of FIG. 21 in a way that the complex PBS 25 of FIG. 21 isseparated into a first complex PBS 251 and a second complex PBS 252.

The first complex PBS 251 has a polarization beam splitter (PBS) surface251A, a polarization characteristic of which is such that it transmitssubstantially 100% of P polarized light and reflects substantially 100%of S polarized light with respect to the first semiconductor laser 20 ofthe 650 nm band. In addition, with respect to the second semiconductorlaser 21 of the 780 nm band, the PBS surface 251A has such apolarization characteristic as to transmit substantially 100% of boththe P and S polarized light.

The second complex PBS 252 has a polarization beam splitter (PBS)surface 252A having such a polarization characteristic as to transmitsubstantially 100% of P polarized light and reflect substantially 100%of S polarized light at least with respect to the second semiconductorlaser 21 of the 780 nm band. With respect to the first semiconductorlaser 20 of the 650 nm band, it has such a polarization characteristicto transmit substantially 100% of both the P and S polarized light.

Further, the wave plate 26 is fixedly bonded to an upper surface of thecomplex PBS 251 and is set to have a thickness which generates a phasedifference acting as a ¼ wave plate with respect to both wavelengths ofthe first semiconductor laser 20 of the 650 nm band and the secondsemiconductor laser 21 of the 780 nm band.

The first hologram element 23 is formed on the transparent substrate291, which diffracts a light beam of the first semiconductor laser 21 soas to guide it to the photosensor 27. The transparent substrate 291 isfixedly bonded to a lower side of the first complex PBS 251 integrally.On the other hand, the second hologram element 24 is formed on thetransparent substrate 292, which diffracts a light beam of the secondsemiconductor laser 21 so as to guide it to the light receiving element27. The transparent substrate 292 is fixedly bonded to a lower side ofthe second complex PBS 252 integrally.

Note that, the structures of the first and second hologram elements 23and 24 and the photosensor 27, and the method for detecting a servosignal are the same as those of FIGS. 21 and 22, thereby omittingexplanation thereof here.

Next, the following will explain the control of the hologram elements 23and 24. First, the second semiconductor laser 21 is caused to emitlight, and the returning light is reflected at the polarization beamsplitter surface 252A of the second complex PBS 252. Thereafter, thereflected light is diffracted at the second hologram element 24 andguided to the photosensor 27. With this state, in order to reduce theFES offset to zero, the transparent substrate 292 (i.e., the secondhologram element 24) to which the complex PBS 252 is integrally andfixedly bonded is controlled by rotation in the θ direction, whilecontrolling the positions in the x and y directions so that the centerof a beam and the center of a hologram coincide.

Here, the returning light 43 of the second semiconductor laser 21 doesnot affect the positions of the first hologram element 23 and thecomplex PBS 251.

Further, the transparent substrate 292 to which the complex PBS 252 isintegrally and fixedly bonded is further fixedly bonded to the laserpackage 28. Thereafter, the first semiconductor laser element 20 iscaused to emit light, the returning light 42 is reflected at thepolarization beam splitter surface 251A of the first complex PBS 251.Thereafter, the reflected light is diffracted at the first hologramelement 24 and guided to the photosensor 27. As above, in order toreduce the FES offset to zero, the transparent substrate 291 (i.e., thefirst hologram element 23) is controlled by rotation in the θ direction,while controlling the positions in the x and y directions so that thecenter of a beam and the center of a hologram coincide.

As described, it is possible to control the positions of beamsrespectively from the two semiconductor lasers 20 and 21 over thephotosensor 27 independently and separately. This makes it possible tonot only attain the same effect as with the integrated laser unit 10 ofFIG. 21 but also control and fix the lamination of a plurality ofcomplex PBSs 251 and 252 and the hologram elements 23 and 24 in a upwardsequence from the bottom on a cap of the laser package 28, therebyimproving mass productivity of the integrated laser unit 10.

The following will describe another embodiment capable of attaining thesame effect as above, configurations of which are shown in FIGS. 24 to29.

In an integrated laser unit 10 of FIG. 24, first and second holograms231 and 241 in the integrated laser unit of FIG. 23 are formed asreflective hologram elements on a reflector surface 251B of the complexPBS 251 and a reflector surface 252B of the second complex PBS 252,respectively.

Further, the three-beam diffraction grating 22 is formed on apolarization beam splitter surface 252A of the second complex PBS 252.This enables the transparent substrates 291 and 292 of FIG. 23 to beeliminated, thus reducing the number of components.

Note that, the control of the first and second hologram elements 231 and241 is the same as that of the first and second hologram elements 23 and24 of FIG. 23.

An integrated laser unit 10 of FIG. 25 adopts the complex PBS 25 havingthe polarization beam splitter surfaces 251A and 252A in the integratedlaser unit 10 of FIG. 23. In addition, the reflector surface 25B hassuch a characteristic as to reflect substantially 100% of light from thefirst semiconductor laser 20 of the 650 nm band and transmit light fromthe second semiconductor laser 21 of the 780 nm band.

The first hologram element 23 is formed on the transparent substrate291. On the other hand, the ‘reflective’ second hologram element 241 isformed on the transparent substrate 292. The transparent substrate 292is disposed on the reflector surface 25B so that the second hologramelement 241 faces the reflector surface 25B of the complex PBS 25.

Next, the control of the hologram elements 23 and 241 will be explained.First, with respect to the first semiconductor laser 20, in order toreduce the FES offset to zero, the complex PBS 25 and the transparentsubstrate 291 (i.e., the first hologram element 23 ) are controlled byrotation in the θ direction, while controlling the positions in the xand y directions so that the center of a beam and the center of ahologram coincide. Further, after fixedly bonding the complex PBS 25 tothe laser package 28, with respect to the second semiconductor laser 21,in order to reduce the FES offset to zero, the transparent substrate 292(i.e., the second hologram element 241) is controlled by rotation andparallel shift on the reflector surface 25B.

An arrangement as shown in FIG. 24 requires a complicated fabricationstep to incorporate a hologram element into the polarization beamsplitter surface of the complex PBS 25. In contrast, in the arrangementof FIG. 25, the second hologram element 241 can be fabricated on the‘plane’ transparent substrate 292, thereby improving mass productivityof the complex PBS 25.

An integrated laser unit 10 of FIG. 26 is provided with a complex PBS25, a configuration of which is a slight modification of that of thecomplex PBS 25 in the integrated laser unit 10 of FIG. 25. In thepresent complex PBS 25, the polarization beam splitter surface 25A hassuch a characteristic as to transmit substantially 100% of P polarizedlight and reflect substantially 100% of S polarized light with respectto the first semiconductor laser 20 of the 650 nm band and the secondsemiconductor laser 21 of the 780 nm band. Further, the polarizationbeam splitter (PBS) surface 25B has such a characteristic as to at leastreflect substantially 100% of S polarized light with respect to thelight of the first semiconductor laser 20 and at least transmitsubstantially 100% of S polarized light with respect to the light of thesecond semiconductor laser 21. Further, a reflector surface 25C has sucha characteristic as to at least transmit substantially 100% of Spolarized light at least with respect to the light of the secondsemiconductor laser 21.

The first hologram element 23 is formed on the transparent substrate291. On the other hand, the ‘reflective’ second hologram element 241 isformed on the transparent substrate is formed on the transparentsubstrate 292. The transparent substrate 292 is provided on thereflector surface 25C of the complex PBS 25. The control of the hologramelements is the same as that of FIG. 25, thus attaining the same effect.

The respective integrated laser units 10 of FIGS. 27 and 28 are providedwith a complex PBS, a configuration of which is a further simplificationof the configuration of the complex PBS 25 in the integrated laser unit10 of FIG. 26. In the present complex PBS 25, a reflector surface 25D ison common ground with the PBS surface 25B and the reflector surface 25Cof the complex PBS 25 shown in FIG. 26. The reflector surface 25D hassuch a characteristic as to reflect substantially 100% of S polarizedlight with respect to the first semiconductor laser 20 of the 650 nmband and transmit substantially 100% of S polarized light with respectto the second semiconductor laser 21 of the 780 nm band. The firsthologram element 23 on one hand is formed on the transparent substrate291, and the ‘reflective’ second hologram element 241 on the other handis formed on the transparent substrate 292. The transparent substrate292 is provided on the reflector surface 25D of the complex PBS 25.

In the integrated laser unit 10 of FIG. 27, the second hologram element241 being a reflective hologram is formed on one side of the transparentsubstrate 292 in contact with the reflector surface 25D of the complexPBS 25. In contrast, in the integrated laser unit 10 of FIG. 28, thesecond hologram element 241 is formed on one side of the transparentsubstrate 292 opposing the side in contact with the reflector surface25D. Compared to the configuration of FIG. 27, the configuration of FIG.28 more easily allows spatial separation between the returning light 42of the first semiconductor laser 20 and the returning light 43 of thesecond semiconductor laser 21, and allows each of the diffractiongratings of the hologram elements 23 and 241 to have a larger pitch,thereby improving mass productivity of the integrated laser unit 10.

Further, in an integrated laser unit 10 of FIG. 29, a ‘transmissive’second hologram element 24 is adopted instead of the second hologramelement 241 according to the configuration of FIG. 27. A reflectorsurface 292B of the transparent substrate 292 having the second hologramelement 24 formed thereon reflects the light which was transmittedthrough a transmission surface 292 A of the transparent substrate 292and diffracted at the second hologram element 24, thereafter guiding thediffracted light to the photosensor 27. With this arrangement, not onlya reflective hologram but also a transmissive hologram can be adopted incombination with the simplified complex PBS 25 having the reflector 25D.

Note that, the control of the hologram elements are the same as that ofFIG. 25, thus attaining the same effect.

Next, the following will describe in detail a Third Embodiment accordingto the present invention with reference to FIGS. 30 to 34. Note that,the same portions as those pertaining to the First Embodiment will begiven the same reference symbols, and explanation thereof will beomitted here.

FIGS. 30 and 31 are schematic diagrams showing an optical pickupaccording to the present embodiment. The optical pickup has the sameconfiguration as with the First Embodiment above described referring toFIG. 1 except for the configuration of the integrated laser unit 10,thereby omitting detailed explanation of the optical pickup here. Thefollowing will explain the present integrated laser unit 10 withreference to FIG. 31.

The integrated laser unit 10 according to the present embodiment is madeup of a first semiconductor laser 20, a second semiconductor laser 21, athree-beam diffraction grating (wavelength selecting diffractiongrating) 32, a first hologram element 33, a second hologram element 34,transparent substrates 35 and 36, and a photosensor 37.

The first semiconductor laser 20 which starts oscillating when awavelength of laser light is in a 650 nm band and the secondsemiconductor laser 21 which starts oscillating when a wavelength oflaser light is in a 780 nm band are adjacently disposed. The three-beamdiffraction grating 32 causes emergence of three beams for trackingcontrol. The first hologram element 33 diffracts respective light beamsof the first and second semiconductor lasers 20 and 21. Further, thesecond hologram element 34 diffracts only a light beam of the secondsemiconductor laser 21 of all the light beams diffracted by the firsthologram element 33, and guides the light beam to a light receivingelement 37. The first hologram element 33 is formed on an upper side ofthe transparent substrate 36. The second hologram element 34 and thediffraction grating 32 are formed on a lower side of the transparentsubstrate 35.

Note that, the structures of light sources of the present optical pickupare the same as those pertaining to the First Embodiment described withreference to FIG. 3, thereby omitting explanation here.

Next, the following will explain a method for playing back differentoptical disks 6A and 6B. Since the method is basically the same as thatof the First Embodiment, the function of the integrated laser unit 10alone will be explained.

For example, when playing back a DVD having a plate thickness of 0.6 mm,a light beam 40 emitted from the first semiconductor laser 20 of the 650nm band is transmitted through the diffraction grating 32, and incidenton the first hologram element 33 so as to be diffracted. Of all thediffracted light, 0th order light is focused on the optical disk 6Ahaving a plate thickness of 0.6 mm by a collimator lens 11 and anobjective lens 5.

Further, the returning light is diffracted at the first hologram element33, and transmitted through the second hologram element 34 so as to befocused on the photosensor 37.

Meanwhile, when playing back a CD having a plate thickness of 1.2 mm, alight beam 41 emitted from the second semiconductor laser element 21 ofthe 780 nm band is split into three beams by the diffraction grating 32,and the beams are incident on the first hologram element 33 so as to bediffracted again. Of all the diffracted light beams, 0th order light isfocused on the optical disk 6B having a plate thickness of 1.2 mm by thecollimator lens 11 and the objective lens 5.

Further, the returning light is diffracted at the first hologram element33, thereafter being further diffracted at the second hologram element34 so as to be focused on the photosensor 37. Here, the functions of awavelength selecting aperture 12 and the three-beam diffraction grating32 are the same as those of the wavelength selecting aperture 12 and thethree-beam diffraction grating 22 of the First Embodiment.

The first hologram element 33 is set to have a groove depth whichdiffracts both light having the wavelength of the first semiconductorlaser 20 and light having the wavelength of the second semiconductorlaser 21. However, since the wavelengths are different, diffractionangles are also different with respect to the respective light beams ofthese wavelengths.

FIG. 32 shows details of diffraction light. In FIG. 32, of all the lightdiffracted at the first hologram element 33, numeral 47 denotes thelight of the first semiconductor laser 20, and numeral 48 denotes thelight of the second semiconductor laser 21. The first hologram element33 is designed to focus the light of the first semiconductor laser 20ideally on a point Q over the photosensor 37.

In that case, diffraction light 48 of the second semiconductor laser 21has a larger diffraction angle than diffraction light 47. In the absenceof the second hologram element 34, the diffraction light 48 follows anoptical path 49, and therefore is focused on a point P which deviatesfrom the ideal point Q over the photosensor 37.

For the shared use of the photosensor 37, focusing on the point Q isessential. Therefore, the second hologram element 34 is provided so asto focus the diffraction light 48 on the point Q by diffracting itagain.

The light of the first semiconductor laser 20 is not affected because ofthe use of 0th order diffraction light (transmission light) of thesecond hologram element 34. As the second hologram element 34,alternatively, a wavelength selective hologram which does not diffractthe light of the first semiconductor laser 20 may be adopted.

Next, the following will explain the structures of the first hologramelement 33 and the photosensor 37, and a method for detecting a servosignal. FIGS. 33 and 34 respectively show the first hologram element 33and the forms of light receiving elements of the photosensor 37.

As shown in FIG. 33, the hologram element 33 is split into three regions33 a to 33 c by a split line 33 l in the x direction corresponding tothe radial direction of the optical disks 6A and 6B and by a split line33 m in the y direction corresponding to the track direction.

The light receiving elements include a 2-division light receivingelement which is divided into light receiving regions 37 a and 37 b by adivision line 37 l, and eight light receiving regions 37 c to 37 j(here, respective output form the light receiving regions are referredto as Sa to Sj).

For example, when playing a DVD back, returning light from the opticaldisk 6A, which was originally emitted from the first semiconductor laser20, is incident on the first hologram element 33.

When a light beam is focused on an information recording side of theoptical disk 6A by the objective lens 5, one of the incident beamsdiffracted at the region 33 a of the first hologram element 33 isfocused on the division line 37 l dividing the light receiving elementinto the light receiving regions 37 a and 37 b. Likewise, lightdiffracted at the region 33 b of the first hologram element 33 isfocused on the light receiving region 37 d, and light diffracted at theregion 33 c of the first hologram element 33 is focused on the lightreceiving region 37 c.

As to a servo signal, as with the method explained in the FirstEmbodiment above, a focus error signal (FES) according to the singleknife edge method can be detected using Sa and Sb by FES=Sa−Sb.

Further, when playing back the optical disk 6A storing pit information,a tracking error signal 1 (TES 1) according to the phase difference(DPD) method can be detected by detecting a change in a signal phasedifference between Sc and Sd.

In the case of the optical disk 6A having grooves formed therein, atracking error signal 2 (TES 2) according to the push-pull method can bedetected by TES 2=Sc−Sd.

Further, a stored information signal (RF signal) can be reproduced byRF=Sa+Sb+Sc+Sd.

Next, the following will explain the case of playing a CD back.Returning light from the optical disk 6B, which was originally emittedfrom the second semiconductor laser 21, is diffracted at the firsthologram element 33. When the diffracted light travels as it is, thelight beam is incident on the optical path 49 shown in FIG. 33.

Here, the same beam is diffracted by the second hologram element 34 soas to be incident on the light receiving element of the photosensor 37as shown in FIG. 34. Light diffracted at the region 33 a of the firsthologram element 33 is focused on the division line 37 l dividing thelight receiving element into the light receiving regions 37 a and 37 b.Likewise, light diffracted at the region 33 b of the first hologramelement 33 is focused on the light receiving region 37 d, and lightdiffracted at the region 33 c of the first hologram element 33 isfocused on the light receiving region 37 c.

The light of the second semiconductor laser 21 is split into a main beamand two sub-beams A and B by the three-beam diffraction grating 32.Therefore, the sub-beams A and B diffracted at the region 33 a arerespectively focused on the light receiving regions 37 f and 37 e, andthe sub-beams A and B diffracted at the region 33 b of the firsthologram element 33 are respectively focused on the light receivingregions 37 j and 37 i. Likewise, the sub-beams A and B diffracted at theregion 33 c of the first hologram element 33 are respectively focused onthe light receiving regions 37 h and 37 g.

A focus error signal (FES), as with the DVD, can be detected byFES=Sa−Sb.

Further, a tracking error signal 6 (TES 6) according to the three-beammethod can be detected byTES 6=(Sf+Sh+Sj)−(Se+Sg+Si).

Further, a tracking error signal 7 (TES 7) according to the differentialpush-pull (DPP) method can be detected byTES 7=(Sd−Sc)−k·((Sj−Sh)+(Si−Sg))

Here, a coefficient k is provided to correct a difference in lightintensity between the main beam and sub-beams. Accordingly, when a ratioof intensity is the main beam:sub-beam A:sub-beam B=a:b:b, thecoefficient k=a/(2b).

Further, a stored information signal (RF signal) can be reproduced byRF=Sa+Sb+Sc+Sd.

Next, the following will explain the control of the hologram elements.As with the First Embodiment above, the FES offset control will beexplained. The control is the same as that of the First Embodiment inprinciple, explanation will be made briefly here.

First, the first semiconductor laser 20 is cased to emit light, and thereturning light is diffracted at the first hologram element 33,thereafter guiding the diffracted light to the photosensor 37. With thisstate, in order to reduce the FES offset to zero, the transparentsubstrate 36 (i.e., the first hologram element 33) is controlled byrotation in the θ direction, while controlling the positions in the xand y directions so that the center of a beam and the center of ahologram coincide.

In that case, since the light 47 is only transmitted through the secondhologram element 34, the position of the transparent substrate 35 is notaffected.

Further, after the position of the transparent substrate 36 is fixed,the second semiconductor laser 21 is caused to emit light so as todiffract the returning light at the first hologram element 33,thereafter guiding the diffracted the light to the second hologramelement 34. Since the first hologram element 33 has been optimized withrespect to the first semiconductor laser 20, it may not ideally besuited to the light of the second semiconductor laser.

Accordingly, the transparent substrate 35 (i.e., the second hologramelement 34) is controlled by rotation in the θ direction and by parallelshifts in the x and y directions so as to control a diffraction angle ofa beam 50 shown in FIG. 32, thereby reducing the FES offset to zero.

As described, it is possible to control the positions of beams from thetwo semiconductor lasers 20 and 21 over the photosensor 37 independentlyand separately. Accordingly, an optimum servo error signal can beobtained by the control even when there are tolerances in the positionalrelationship between laser chips, or in the position and the angle ofthe laser package 28, the photosensor 37 and the hologram elements 33and 34.

Further, the following will explain a Fourth Embodiment according to thepresent invention with reference to FIGS. 35 and 36. This relates to anoptical pickup adopting the integrated laser unit 10 described in theFirst to Third Embodiments above, and in particular a configuration torealize an optical pickup for recording and playing back CD-Rs andCD-RWs by using a high-power laser as the second semiconductor laser 21.

The detection of a servo signal can be realized, as with the firstEmbodiment described with reference to FIG. 15, by utilizing the DPPmethod for a TES. However, in an optical system shown in FIGS. 1 and 30,in order to correct a difference in disk thickness, adopted are thewavelength selecting aperture 12 and the special objective lens 5 havinga partially corrected aspherical form.

In that case, for example, assuming that a NA of the collimator lens 11is designed so as to match a DVD system having strict focusingconditions, a virtual NA of the collimator lens 11 for a CD systembecomes small due to the wavelength selecting aperture 12.

Recording disks such as CD-Rs and the like require a large quantity ofemission light from the objective lens. Therefore, the NA of thecollimator lens 11 is made larger than an optical pickup forplayback-only CD-ROMs, thereby improving utilization efficiency withrespect to light from semiconductor laser light sources.

The optical pickups according to the First to Third Embodiments alladopt the integrated laser unit 10 mounting the two adjacently disposedsemiconductor lasers 20 and 21 having different wavelengths. Therefore,when using an optical system of the CD system, the NA of the collimatorlens 11 cannot be set flexibly.

Accordingly, in the present embodiment, as an optical pickup forrecording and playing back CD-Rs and CD-RWS, configurations as shown inFIGS. 35 and 36 are adopted.

Namely, FIG. 35, as with FIG. 1, shows an optical system in the case ofplaying a DVD back, where light emitted from the integrated laser unit10 is changed into parallel light by the collimator lens 11 so as to befocused on the optical disk 6A by a DVD-only objective lens 38.

The reflected light, travelling again through the same opticalcomponents in an outward travel path, is focused on a photosensor (thephotosensor 27, 30, 31 or 37 described above) in the integrated laserunit 10.

On the other hand, FIG. 36 shows an optical system in the case ofrecording and playing back CD-Rs and CD-RWs, where, as above, lightemitted from the integrated laser unit 10 is changed into parallel lightby the collimator lens 11, thereafter being focused on the optical disk6B by a CD-only objective lens 39.

The reflected light, travelling again through the same opticalcomponents in the outward travel path, is focused on the photosensor inthe integrated laser unit 10.

Here, by switching between the objective lenses 38 and 39, the virtualNA of the collimator lens 11 is enlarged in the CD system. By setting aneffective diameter of the CD system objective lens 39 larger than thatof the DVD-only objective lens 38, it is possible to improve opticalutilization efficiency in the CD system by using the shared collimatorlens 11.

The switch between the objective lenses 38 and 39 is realized by using asliding-axis type two-lens actuator 44 or the like. The virtual NA ofthe collimator lens is preferably set at about 0.1 in the DVD system orabout 0.13 to 0.15 in the CD system.

It is also possible to switch collimator lenses by sharing an objectivelens. Switch like this, however, requires to further provide a drivingsystem in addition to the objective lens actuator. This increases costs,upsizes a pickup, and further causes adverse effects such as deviationof an optic axis, thus being unrealistic for the actual use.

In the respective optical pickup optical systems shown in FIGS. 35 and36, explanation has been made through the case where the integratedlaser unit 10 according to the First Embodiment of the present inventionis mounted, though not necessarily limited to this. When using theintegrated laser unit 10 according to the Second Embodiment of thepresent invention, for example, since a hologram element is not disposedin the middle of an outward travel path, it is possible to improve anefficiency of emission from an objective lens with respect to both lightbeams from the first and second semiconductor laser elements 20 and 21.Consequently, this realizes not only the recording and playback of CD-Rsand CD-RWs by the high-power second semiconductor laser 21. Namely, byadditionally mounting a high-power first semiconductor laser 20, it isalso possible to realize the recording and playback of DVD-Rs, DVD-RWs,DVD-RAMs and the like.

The embodiments and concrete examples of implementation discussed in theforegoing detailed explanation serve solely to illustrate the technicaldetails of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

1. An optical pickup, comprising: a first light source for generating alight beam having a first wavelength; a second light source forgenerating a light beam having a second wavelength different from thefirst wavelength; a lens system for focusing the two light beams on anoptical disk; a photosensor for sensing reflection light beams from theoptical disk; an optical path splitting element for splitting opticalpaths of the two reflection light beams having different wavelengths;and first and second hologram elements for respectively diffracting atleast one of the two light beams of the different wavelengths which weresplit by the optical path splitting element, and guiding the thusdiffracted light to the shared photosensor, wherein one of the first andsecond hologram elements is disposed in an optical path being an outwardtravel path between the light sources and the lens system, and the otheris disposed in an optical path being a return travel path between theoptical path splitting element and the photosensor through which onlyreturning light travels.
 2. The optical pickup set forth in claim 1,wherein the optical path splitting element is a polarization beamsplitter.
 3. The optical pickup set forth in claim 2, wherein thepolarization beam splitter has different polarization characteristicswith respect to a light beam of the first wavelength and a light beam ofthe second wavelength.
 4. The optical pickup set forth in claim whereinthe polarization beam splitter has such characteristics as to transmitsubstantially full P polarized light and reflect substantially full Spolarized light with respect to the light beam of the first wavelength,and transmit substantially full P and S polarized light with respect tothe light beam of the second wavelength.
 5. The optical pickup set forthin claim 4, further comprising a wave plate which is disposed in theoptical path between the polarization beam splitter and the lens systemso as to generate a ¼ wavelength phase difference with respect to thelight beam of the first wavelength and an arbitrary phase differencewith respect to the light beam of the second wavelength.
 6. The opticalpickup set forth in claim 5, wherein the wave plate is integrally andfixedly bonded to an emission side of the polarization beam splitter. 7.The optical pickup set forth in claim 2, wherein the polarization beamsplitter has the same polarization characteristic with respect to thelight beams of the first wavelength and the second wavelength.
 8. Theoptical pickup set forth in claim 7, wherein the polarization beamsplitter has such a characteristic as to transmit substantially full Ppolarized light and reflect substantially full S polarized, with respectto the light beams of the first wavelength and the second wavelength. 9.The optical pickup set forth in claim 8, further comprising a wave platewhich is disposed in the optical path between the polarization beamsplitter and the lens system so as to generate a ¼ wavelength phasedifference with respect to the light beam of the first wavelength and a½ wavelength phase difference with respect to the light beam of thesecond wavelength.
 10. The optical pickup set forth in claim 2, whereina shared 2-division light receiving element is used to detect a focuserror signal according to a single knife edge method from respectivesemicircular beams of either of the light beams of the first wavelengthand the second wavelength, the semicircular beams being split by a splitline in a direction corresponding to a direction of the radius of theoptical disk.
 11. The optical pickup set forth in claim 10, wherein adirection of the division line of the 2-division light receiving elementis set so that, in a focus error signal due to the light beam of thefirst wavelength diffracted by the first hologram element and a focuserror signal due to the light beam of the second wavelength diffractedby the second hologram element, an offset resulted from variations inwavelength of the first and second light sources is cancelled.
 12. Theoptical pickup set forth in claim 10, wherein a position of the divisionline of the 2-division light receiving element is set so that, in afocus error signal due to the light beam of the first wavelengthdiffracted by the first hologram element and a focus error signal due tothe light beam of the second wavelength diffracted by the secondhologram element, an offset resulted from variations in wavelength ofthe first and second light sources is cancelled.
 13. The optical pickupset forth in claim 2, wherein: the first hologram element is used todetect a tracking error signal according to a phase difference method ora push-pull method, and the second hologram element is used to detect atracking error signal according to a three-beam method or a differentialpush-pull method.
 14. The optical pickup set forth in claim 2, whereinthe photosensor is disposed lest diffraction light of the light beam ofthe second wavelength from the first hologram element and diffractionlight of the light beam of the first wavelength from the second hologramelement should enter the photosensor.
 15. The optical pickup set forthin claim 2, further comprising a wavelength selecting diffractiongrating for transmitting the substantially full light beam of the firstwavelength and splitting the light beam of the second wavelength intothree beams including 0th order light and ±1st order light, thewavelength selecting diffraction grating being disposed between thefirst and second light sources and the second hologram element.
 16. Theoptical pickup set forth in claim 1, wherein: the first hologram elementis set to have a groove depth such that ±1st order diffractionefficiency is largest with respect to a light beam of the firstwavelength, and the second hologram element is set to have a groovedepth such that a product of ±1st order diffraction efficiency and 0thorder diffraction efficiency is largest with respect to a light beam ofthe second wavelength.
 17. The optical pickup set forth in claim 1,where the first light source is a semiconductor laser of a 650 nm band,and the second light source is a semiconductor laser of a 780 nm band.18. The optical pickup set forth in claim 1, further comprising twoobjective lenses having different effective diameters respectivelycorresponding to the first and second light sources.
 19. An opticalpickup, comprising: a first light source for generating a light beamhaving a first wavelength; a second light source for generating a lightbeam having a second wavelength different from the first wavelength; alens system for focusing the two light beams on an optical disk; aphotosensor for sensing reflection light beams from the optical disk; anoptical path splitting element for splitting optical paths of the tworeflection light beams having different wavelengths; and first andsecond hologram elements for respectively diffracting at least one ofthe two light beams of the different wavelengths which were split by theoptical path splitting element, and guiding the thus diffracted light tothe shared photosensor, wherein the optical path splitting element andat least one of the first and second hologram elements are separatelyprovided so that a position of either of the two reflecting light beamscan independently be controlled over the photosensor by separatelycontrolling each of the elements, wherein one of the first and secondhologram elements is disposed in an optical path being an outward travelpath between the light sources and the lens system, and the other isdisposed in an optical path being a return travel path between theoptical oath splitting element and the photosensor through which onlyreturning light travels.
 20. The optical pickup set forth in claim 19,wherein the optical path splitting element is a polarization beamsplitter.
 21. The optical pickup set forth in claim 20, wherein thepolarization beam splitter has different polarization characteristicswith respect to a light beam of the first wavelength and a light beam ofthe second wavelength.
 22. The optical pickup set forth in claim 20,wherein the polarization beam splitter has such characteristics as totransmit substantially full P polarized light and reflect substantiallyfull S polarized light with respect to the light beam of the firstwavelength, and transmit substantially full P and S polarized light withrespect to the light beam of the second wavelength.
 23. The opticalpickup set forth in claim 22, further comprising a wave plate which isdisposed in the optical path between the polarization beam splitter andthe lens system so as to generate a ¼ wavelength phase difference withrespect to the light beam of the first wavelength and an arbitrary phasedifference with respect to the light beam of the second wavelength. 24.The optical pickup set forth in claim 20, wherein the polarization beamsplitter has the same polarization characteristic with respect to thelight beams of the first wavelength and the second wavelength.
 25. Theoptical pickup set forth in claim 24, wherein the polarization beamsplitter has such a characteristic as to transmit substantially full Ppolarized light and reflect substantially full S polarized, with respectto the light beams of the first wavelength and the second wavelength.26. The optical pickup set forth in claim 25, further comprising a waveplate which is disposed in the optical path between the polarizationbeam splitter and the lens system so as to generate a ¼ wavelength phasedifference with respect to the light beam of the first wavelength and a½ wavelength phase difference with respect to the light beam of thesecond wavelength.
 27. The optical pickup set forth in claim 26, whereinthe wave plate is integrally and fixedly bonded to an emission side ofthe polarization beam splitter.
 28. The optical pickup set forth inclaim 20, wherein a shared 2-division light receiving element is used todetect a focus error signal according to a single knife edge method fromrespective semicircular beams of either of the light beams of the firstwavelength and the second wavelength, the semicircular beams being splitby a split line in a direction corresponding to a direction of theradius of the optical disk.
 29. The optical pickup set forth in claim28, wherein a direction of the division line of the 2-division lightreceiving element is set so that, in a focus error signal due to thelight beam of the first wavelength diffracted by the first hologramelement and a focus error signal due to the light beam of the secondwavelength diffracted by the second hologram element, an offset resultedfrom variations in wavelength of the first and second light sources iscancelled.
 30. The optical pickup set forth in claim 28, wherein aposition of the division line of the 2-division light receiving elementis set so that, in a focus error signal due to the light beam of thefirst wavelength diffracted by the first hologram element and a focuserror signal due to the light beam of the second wavelength diffractedby the second hologram element, an offset resulted from variations inwavelength of the first and second light sources is cancelled.
 31. Theoptical pickup set forth in claim 20, wherein: the first hologramelement is used to detect a tracking error signal according to a phasedifference method or a push-pull method, and the second hologram elementis used to detect a tracking error signal according to a three-beammethod or a differential push-pull method.
 32. The optical pickup setforth in claim 20, wherein the photosensor is disposed lest diffractionlight of the light beam of the second wavelength from the first hologramelement and diffraction light of the light beam of the first wavelengthfrom the second hologram element should enter the photosensor.
 33. Theoptical pickup set forth in claim 20, further comprising a wavelengthselecting diffraction grating for transmitting the substantially fulllight beam of the first wavelength and splitting the light beam of thesecond wavelength into three beams including 0th order light and ±1storder light, the wavelength selecting diffraction grating being disposedbetween the first and second light sources and the second hologramelement.
 34. The optical pickup set forth in claim 19, wherein: thefirst hologram element is set to have a groove depth such that ±1storder diffraction efficiency is largest with respect to a light beam ofthe first wavelength, and the second hologram element is set to have agroove depth such that a product of ±1st order diffraction efficiencyand 0th order diffraction efficiency is largest with respect to a lightbeam of the second wavelength.
 35. The optical pickup set forth in claim19, where the first light source is a semiconductor laser of a 650 nmband, and the second light source is a semiconductor laser of a 780 nmband.
 36. The optical pickup set forth in claim 35, wherein at least oneof the first and second light sources is a high-power laser so as toenable recording and playback with respect to the optical disk.
 37. Theoptical pickup set forth in claim 19, further comprising two objectivelenses having different effective diameters respectively correspondingto the first and second light sources.